Annual report pursuant to Section 13 and 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details)

v2.4.1.9
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Site Contingency [Line Items]    
Payments for Environmental Liabilities $ 3.1us-gaap_PaymentsForEnvironmentalLiabilities $ 8.2us-gaap_PaymentsForEnvironmentalLiabilities
Accrual for Environmental Loss Contingencies 5.5us-gaap_AccrualForEnvironmentalLossContingencies 8.4us-gaap_AccrualForEnvironmentalLossContingencies
Mine Reclamation and Closing Liability, current and noncurrent 255.7clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent 312.6clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent
Total environmental and mine closure obligations 261.2clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrentAndNoncurrent 321.0clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrentAndNoncurrent
Current Fiscal Year End Date --12-31  
U.S. Iron Ore [Member]    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 5clf_MineClosureObligationsNumberOfMines
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember 		 
Eastern Canadian Iron Ore [Member]    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 2clf_MineClosureObligationsNumberOfMines
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember 		 
Asia Pacific Iron Ore [Member]    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 1clf_MineClosureObligationsNumberOfMines
/ us-gaap_StatementBusinessSegmentsAxis
= clf_AsiaPacificIronOreMember 		 
North American Coal [Member]    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 2clf_MineClosureObligationsNumberOfMines
/ us-gaap_StatementBusinessSegmentsAxis
= clf_NorthAmericanCoalMember 		 
LTV Steel Mining Company [Member]    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) 1clf_MineClosureObligationsNumberOfMines
/ clf_MineClosureAxis
= clf_LTVSteelMiningCompanyMember 		 
Rio Tinto Mine Site [Member]    
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies 2.5us-gaap_AccrualForEnvironmentalLossContingencies
/ us-gaap_EnvironmentalRemediationSiteAxis
= clf_RioTintoMineSiteMember 		5.3us-gaap_AccrualForEnvironmentalLossContingencies
/ us-gaap_EnvironmentalRemediationSiteAxis
= clf_RioTintoMineSiteMember
Loss Contingency, Range of Possible Loss, Maximum   12.2us-gaap_LossContingencyRangeOfPossibleLossMaximum
/ us-gaap_EnvironmentalRemediationSiteAxis
= clf_RioTintoMineSiteMember
Rio Tinto Mine Site [Member] | Rio Tinto Working Group RTWG [Member]    
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies   $ 29.0us-gaap_AccrualForEnvironmentalLossContingencies
/ us-gaap_CounterpartyNameAxis
= clf_RioTintoWorkingGroupRtwgMember
/ us-gaap_EnvironmentalRemediationSiteAxis
= clf_RioTintoMineSiteMember